JP2006287167A - Rubber composition for radio wave absorbent and radio wave absorbent sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin radio wave absorbent sheet displaying an excellent radio absorbent performance and having excellent environmental resistance such as ozone resistance, weather resistance, and heat resistance. <P>SOLUTION: There is provided a rubber composition for radio wave absorbent where 200 to 1,500 pts.wt. of carbonyl iron is compounded with 100 pts.wt. of rubber component. The rubber component includes 5 pts.wt. or more of weather resistant rubber in the 100 pts.wt. of rubber component, and preferably, further includes 1 to 10 pts.wt. of carbon black. By containing a prescribed amount of carbonyl iron in the rubber component, a significantly excellent radio absorbent performance can be obtained, and thus the thickness of the radio wave absorbent sheet for obtaining required amount of radio wave absorption can be made small. By containing the weather resistant rubber in the rubber component; environmental resistance such as ozone resistance, weather resistance, and heat resistance can be improved. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a rubber composition for a radio wave absorber and a radio wave absorption sheet, and in particular, it is thin and exhibits excellent radio wave absorption performance, and also has excellent radio wave absorption such as ozone resistance, weather resistance, and heat resistance. The present invention relates to a rubber composition for a radio wave absorber capable of providing a sheet, and a radio wave absorber sheet formed by molding the rubber composition for a radio wave absorber into a sheet shape.

  In recent years, with the spread of ETC (Electronic Toll Collection System), OA equipment, communication equipment, etc., electromagnetic waves generated from these equipment have been regarded as problems. That is, there are concerns about the influence of electromagnetic waves on the human body, and malfunctions of precision equipment due to electromagnetic waves are problematic.

  Therefore, various radio wave absorbing materials have been developed for various electronic devices in order to prevent the emission of electromagnetic waves to the outside, absorb the electromagnetic waves from the outside, and prevent mutual interference. Many of them are obtained by dispersing a powder of a magnetic material such as ferrite, permalloy, or sendust in a rubber or synthetic resin matrix and molding the powder into an appropriate shape. The shape is generally a sheet that is convenient for use. In such a radio wave absorbing sheet, it is desired that a thinner sheet exhibits better radio wave absorbing ability. In addition, it can be easily applied to devices having complicated shapes and large-sized devices, and is desired to be excellent in flexibility in order to be hard to break and have high durability.

  The ability to absorb radio waves is usually expressed using radio wave absorption. In order to increase the amount of radio wave absorption, it is basically necessary to increase the amount of magnetic material present in the radio wave path. However, the presence of more magnetic material in the thin sheet, that is, the high filling of the magnetic material powder into the molding material of the sheet results in a great loss of flexibility of the sheet. On the other hand, when the magnetic material is low blended, it is necessary to increase the thickness of the sheet in order to obtain the desired radio wave absorption performance.

  As a rubber composition for a radio wave absorber that can provide a radio wave absorbing sheet that is thin and has excellent radio wave absorption performance and is excellent in flexibility, the applicant of the present invention firstly added 300 parts by weight of carbonyl iron to 100 parts by weight of the rubber component. A rubber composition for a radio wave absorber formed by blending 1500 parts by weight was proposed (Japanese Patent Application No. 2004-346500, hereinafter referred to as “prior application”).

According to the prior application, by blending a predetermined amount of carbonyl iron with a rubber component such as natural rubber, it is possible to obtain a remarkably superior radio wave absorption performance. The thickness of the absorbent sheet can be reduced. In addition, a rubber component such as natural rubber blended with carbonyl iron has an excellent effect of being low in hardness and remarkably excellent in flexibility, thus having good workability, being hard to break, and excellent in durability. .
Japanese Patent Application No. 2004-346500

  The radio wave absorbing sheet is used continuously for a long period of time. Therefore, it is desired to have excellent environmental resistance such as ozone resistance, weather resistance and heat resistance. The rubber composition for radio wave absorbers of the application has the disadvantage that it is poor in environmental resistance such as ozone resistance, weather resistance, heat resistance, etc., so that it tends to age with time, resulting in deterioration of physical properties and easy destruction. .

  The present invention solves the above-mentioned problems of the prior application, and is capable of providing a radio wave absorbing sheet that is thin and exhibits excellent radio wave absorption performance, and is also excellent in environmental resistance such as ozone resistance, weather resistance, and heat resistance. Absorber rubber composition and sheet-shaped rubber composition for radio wave absorbers, which are formed into a sheet, and have excellent resistance to the environment such as ozone resistance, weather resistance, and heat resistance, and radio wave absorption performance The purpose is to provide.

  The rubber composition for a radio wave absorber according to the present invention (Claim 1) is a rubber composition for a radio wave absorber obtained by blending 200 to 1500 parts by weight of carbonyl iron with 100 parts by weight of a rubber component. 5 parts by weight or more of weather resistant rubber is contained in parts by weight.

  The rubber composition for a radio wave absorber according to claim 2, wherein the weather resistant rubber is butyl rubber, chloroprene rubber, ethylene propylene rubber, ethylene propylene diene rubber, chlorinated polyethylene rubber, acrylonitrile butadiene rubber, chlorinated butyl rubber. And one or more selected from the group consisting of brominated butyl rubber and chlorosulfonated polyethylene rubber.

  The rubber composition for a radio wave absorber according to claim 3 is characterized in that in claim 1 or 2, the rubber component contains 20 parts by weight or more of weather resistant rubber in 100 parts by weight of the rubber component.

  The rubber composition for a radio wave absorber according to claim 4 is characterized in that, in any one of claims 1 to 3, the rubber component is composed of a weather resistant rubber and a general-purpose rubber other than the weather resistant rubber. .

  The rubber composition for a radio wave absorber according to claim 5 is characterized in that, in claim 4, the general-purpose rubber is one or more selected from the group consisting of natural rubber, styrene butadiene rubber, isoprene rubber, and butadiene rubber. It is characterized by.

  The rubber composition for a radio wave absorber according to claim 6 is characterized in that, in claim 5, natural rubber and ethylene propylene rubber are used as the rubber component, and natural rubber / ethylene propylene rubber = 95/5 to 5/95 (weight ratio). It is characterized by including in proportion.

  The rubber composition for a radio wave absorber according to claim 7 is the rubber composition according to claim 6, wherein natural rubber and ethylene propylene rubber are used as the rubber component, natural rubber / ethylene propylene rubber = 80/20 to 55/45 (weight ratio). It is characterized by including in proportion.

  The rubber composition for a radio wave absorber according to claim 8 is characterized in that, in any one of claims 1 to 7, carbon black is further contained in an amount of 1 to 10 parts by weight with respect to 100 parts by weight of the rubber component.

  A rubber composition for a radio wave absorber according to a ninth aspect is characterized in that, in any one of the first to eighth aspects, 300 to 1000 parts by weight of carbonyl iron is blended with 100 parts by weight of the rubber component.

  The rubber composition for a radio wave absorber according to claim 10 is characterized in that, in any one of claims 1 to 9, the carbonyl iron has an average particle diameter of 1 to 10 μm.

  A rubber composition for a radio wave absorber according to claim 11 is obtained by adding and kneading a general-purpose rubber after adding and kneading a filler such as carbonyl iron to a weather-resistant rubber according to any one of claims 4 to 10. It is characterized by being able to.

  The rubber composition for a radio wave absorber according to claim 12 is obtained by adding and kneading a filler such as carbonyl iron to ethylene propylene rubber according to claim 11 and then adding and kneading natural rubber.

  The radio wave absorption sheet of the present invention (Claim 13) is characterized by being formed by molding the radio wave absorber rubber composition according to any one of Claims 1 to 12 into a sheet shape.

  According to the present invention, by adding a predetermined amount of carbonyl iron to the rubber component, it is possible to obtain a remarkably excellent radio wave absorption performance, and thereby the thickness of the radio wave absorption sheet for obtaining the required radio wave absorption amount. The thickness can be reduced.

  When general-purpose rubber such as natural rubber or styrene-butadiene rubber is used as a rubber component, the environmental resistance such as ozone resistance, weather resistance, and heat resistance is inferior. By blending 5 parts by weight or more in the part, environmental resistance such as ozone resistance, weather resistance and heat resistance can be improved.

  Therefore, according to the present invention, it is possible to easily take an electromagnetic wave shielding measure excellent in long-term durability by using a high-performance electromagnetic wave absorbing sheet that is thin and excellent in environmental resistance such as ozone resistance, weather resistance, and heat resistance. it can.

  In the present invention, the weather resistant rubber is preferably blended in an amount of 20 parts by weight or more in 100 parts by weight of the rubber component. The weather resistant rubber is selected from the group consisting of butyl rubber, chloroprene rubber, ethylene propylene rubber, ethylene propylene diene rubber, chlorinated polyethylene rubber, acrylonitrile butadiene rubber, chlorinated butyl rubber, brominated butyl rubber and chlorosulfonated polyethylene rubber. A seed | species or 2 or more types is mentioned.

  By using weather resistant rubber as the rubber component, the environmental resistance can be improved, but on the other hand, flexibility and workability may be impaired. Therefore, in this case, it is preferable to use general-purpose rubber as a rubber component to ensure necessary flexibility and processability. Examples of the general-purpose rubber include one or more selected from the group consisting of natural rubber, styrene butadiene rubber, isoprene rubber, and butadiene rubber.

  In the present invention, the light resistance and weather resistance can be further improved by further blending a predetermined amount of carbon black. Further, by setting the blending amount of carbonyl iron to 100 parts by weight of the rubber component to 300 to 1000 parts by weight, the radio wave absorption performance can be more reliably improved. In particular, a weather resistant rubber and a general-purpose rubber are used as the rubber component. When used in combination, it is possible to achieve both electromagnetic wave absorption performance and flexibility and workability. Carbonyl iron having an average particle diameter of 1 to 10 μm is preferable.

  By the way, in blend blending of general-purpose rubber such as natural rubber and weather-resistant rubber such as ethylene propylene rubber, dispersibility may be lowered when the filler is highly blended. However, this problem can be solved by adding and kneading a filler such as carbonyl iron in advance to a weather resistant rubber such as ethylene propylene rubber and then adding and kneading a general-purpose rubber such as natural rubber.

  Hereinafter, embodiments of the rubber composition for a radio wave absorber and the radio wave absorption sheet of the present invention will be described in detail.

  The rubber composition for a radio wave absorber of the present invention contains a predetermined amount of carbonyl iron in a rubber component containing 5 phr of a weather resistant rubber, that is, a rubber component containing 5 parts by weight or more of a weather resistant rubber in 100 parts by weight of the rubber component. It will be.

  In the rubber composition for a radio wave absorber of the present invention, examples of the weather resistant rubber include butyl rubber, chloroprene rubber, ethylene propylene rubber, ethylene propylene diene rubber, chlorinated polyethylene rubber, acrylonitrile butadiene rubber, chlorinated butyl rubber, brominated butyl rubber. And chlorosulfonated polyethylene rubber. These weather resistant rubbers may be used alone or in combination of two or more.

  These weather-resistant rubbers are particularly preferably contained in an amount of 20 parts by weight or more in 100 parts by weight of the rubber component in order to sufficiently enhance the environmental resistance.

  As described above, by using a weather resistant rubber as a rubber component, environmental resistance can be improved, but on the other hand, flexibility and workability may be impaired. Therefore, in this case, it is preferable to use general-purpose rubber as a rubber component to ensure necessary flexibility and processability. Examples of the general-purpose rubber include natural rubber, styrene-butadiene rubber, isoprene rubber, butadiene rubber, and the like. These general-purpose rubbers may be used alone or in combination of two or more.

  The blending ratio of the general-purpose rubber and the weather-resistant rubber in the rubber component is appropriately determined according to the required characteristics of the rubber composition for the radio wave absorber, and the general-purpose rubber / weather-resistant rubber = 95/5 to 5/95 (weight ratio). In particular, in the range of 80/20 to 55/45 (weight ratio), when the environmental resistance is important, the weather resistant rubber is prominent, and when the flexibility and workability are important, the general-purpose rubber It is preferable to blend more frequently.

  In the present invention, the rubber component includes other rubbers other than the above weather resistant rubber and general-purpose rubber, such as bromide (isobutylene-4-methylstyrene copolymer), acrylic rubber, fluorine rubber latex, silicone rubber latex, urethane. 1 type or 2 types or more such as rubber latex, and further 1 type or 2 types or more of thermoplastic elastomers such as SBS (styrene-butadiene-styrene) and SEBS (styrene- (ethylene-butadiene) -styrene) are used in combination. May be.

  In particular, when a natural rubber and a weather-resistant rubber such as ethylene propylene rubber are used in combination as a rubber component, a rubber composition for a radio wave absorber having excellent flexibility and processability as well as environmental resistance is obtained. Can do. In this case, if the proportion of natural rubber in the rubber component is large and the proportion of weathering rubber such as ethylene propylene rubber is small, ozone resistance, weather resistance, heat resistance due to the use of weathering rubber such as ethylene propylene rubber, etc. However, if the proportion of natural rubber is small and the proportion of weathering rubber such as ethylene propylene rubber is large, the flexibility and processability of natural rubber can be reduced. Such effects cannot be obtained sufficiently. Therefore, the ratio of the natural rubber and the weather resistant rubber such as ethylene propylene rubber in the rubber component is natural rubber / weather resistant rubber such as ethylene propylene rubber = 95 / 5-5 / 95 (weight ratio), 80 / 20-55. / 45 (weight ratio) is preferable.

  In this case, as the rubber component, in addition to natural rubber and weather resistant rubber such as ethylene propylene rubber, various synthetic rubbers such as styrene butadiene rubber, butadiene rubber, isoprene rubber, bromide (isobutylene-4-methylstyrene) Copolymer), acrylic rubber, fluororubber latex, silicone rubber latex, urethane rubber latex and the like, or more, SBS (styrene-butadiene-styrene), SEBS (styrene- (ethylene-butadiene) -styrene ) Etc. may be used in combination. However, in order to sufficiently obtain the above effect by using natural rubber and weather resistant rubber such as ethylene propylene rubber as the rubber component, the amount of rubber components other than natural rubber and weather resistant rubber such as ethylene propylene rubber used. Is preferably 20 parts by weight or less, more preferably 10 parts by weight or less, based on 100 parts by weight of the total rubber component, and in particular, the rubber component is preferably composed of natural rubber and weather resistant rubber such as ethylene propylene rubber. .

  The carbonyl iron powder used in the present invention is an almost granular fine powder having an average particle diameter of about 1 to 10 μm obtained by pyrolyzing iron carbonylated as an organometallic compound. It is an excellent radio wave absorptive material that absorbs electromagnetic waves as resistance loss due to its conductivity.

  In the present invention, if the amount of carbonyl iron added to the rubber component is small, sufficient radio wave absorption performance cannot be obtained, and if it is large, the flexibility tends to decrease. Accordingly, carbonyl iron is blended in an amount of 200 to 1500 parts by weight, preferably 300 to 1000 parts by weight, based on 100 parts by weight of the rubber component.

  In the rubber composition for a radio wave absorber of the present invention, by further blending carbon black, it is possible to obtain a further effect of improving light resistance and weather resistance due to the ultraviolet absorption effect of carbon black. The compounding amount of carbon black is preferably 1 to 10 parts by weight with respect to 100 parts by weight of the rubber component. If the amount of carbon black is less than this range, the above-mentioned effect due to the combined use of carbon black cannot be obtained sufficiently, and if it is too much, the radio wave absorption performance is affected, which is not preferable.

  In addition to carbonyl iron and carbon black, the rubber composition for a radio wave absorber of the present invention includes various vulcanizing agents, vulcanizing aids, and other vulcanization accelerating aids, softeners, anti-aging agents, Various additives usually used in rubber compositions such as an agent, a release agent, a curing agent, a foaming agent, and a colorant are blended and added as necessary.

  Examples of the vulcanizing agent include sulfur, organic sulfur-containing compounds, metal oxides (such as zinc white and magnesia), and organic peroxides (benzoyl peroxide), but are not limited thereto.

Examples of the vulcanization accelerator include the following, but are not limited thereto.
[Aldehyde / Ammonia]
AC: Acetaldehyde ammonia Hexa: Hexamethylene tetramine [aldehyde amines]
K: Acetaldehyde / aniline 8: Butyraldehyde / aniline [guanidines]
D (DPG): Diphenyl guanidine DT (DOTG): Di-o-tolyl guanidine BG: o-tolyl biguanide [thiazoles]
M: 2-mercapto benzothiazole DM: dibenzothiazyl disulfide MZ: zinc salt of 2-mercapto benzothiazole CZ: N-cyclohexyl-2-benzothiazole sulfenamide NOBS: N-oxydiethylene-2- Benzothiazylsulfenamide [Thiurams]
TT: Tetramethyl, thiuram, di-sulfide TS: Tetramethyl-thiuram, mono-sulfide [dithio-carbamates]
P: Pipecoline / Methylpentamethylene / Di-thiocarbamate PZ: Dimethyl / dithiocarbamate EZ: Diethyl / dithiocarbamate BZ: Dibutyl / dithiocarbamate PX: Ethyl / phenyl dithiocarbamate SDC: Diethyl / dithiocarbamate TP: Sodium dibutyl dithiocarbamate

  In the rubber composition for a radio wave absorber of the present invention, usually, the vulcanizing agent is 0.1 to 10 parts by weight, particularly 1 to 5 parts by weight, and the vulcanization accelerator is a rubber component with respect to 100 parts by weight of the rubber component. It is preferable to mix | blend so that it may become 0.1-10 weight part with respect to 100 weight part, especially 1-5 weight part.

  In addition, the rubber composition for a radio wave absorber of the present invention may contain a magnetic material other than carbonyl iron, such as ferrite powder, but in this case, the radio wave absorber has excellent radio wave absorption performance. In obtaining, it is preferable that the other magnetic material is 1700% by weight or less, particularly 1200% by weight or less based on the total of carbonyl iron and the other magnetic material.

  In order to prepare such a rubber composition for a radio wave absorber according to the present invention, in particular, a rubber composition for a radio wave absorber containing a general-purpose rubber and a weather resistant rubber as rubber components, a weather resistant rubber such as ethylene propylene rubber is prepared in advance. In addition, after adding and kneading a filler such as carbonyl iron (carbon black is also included in the filler when carbon black is used), a general-purpose rubber such as natural rubber is added and kneaded. It is preferable to obtain a rubber composition for a radio wave absorber excellent in uniformity and workability of radio wave absorption performance by uniformly dispersing a filler such as carbonyl iron. In this case, carbonyl iron may be added and kneaded in two or more times.

  The radio wave absorbing sheet of the present invention is obtained by vulcanizing and molding such a rubber composition for a radio wave absorber of the present invention into a sheet, and the radio wave absorbing sheet of the present invention is particularly JIS K6253 (type It is preferably designed such that the hardness according to A) is 80 degrees or less and the 5.8 GHz radio wave absorption amount at a thickness of 3 mm or less is 15 dB or more. The rubber composition for a radio wave absorber of the present invention is JIS K6259. In the ozone resistance measured by the method, it is desirable that no cracks occur after 100 hours.

  Such a radio wave absorption sheet of the present invention may be used by being laminated with a radio wave absorption sheet of another configuration, and if necessary, an adhesive layer or an adhesive layer may be formed on the back side, or the front side For example, a design layer may be formed on the surface and used.

  Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.

Examples 1 to 5, Comparative Examples 1 and 2
A radio wave absorber rubber composition was prepared with the rubber composition shown in Table 1, and the radio wave absorber rubber composition was vulcanized to obtain a radio wave absorber sheet having the thickness shown in Table 1. In Examples 1 to 4 and Comparative Example 2, components other than natural rubber were added and kneaded in advance to ethylene propylene rubber, and then natural rubber was added and kneaded (two-step kneading). In Comparative Example 1 and Example 5, all components were added and kneaded at one time (one-step kneading).

The following evaluation was performed for each of the radio wave absorber rubber composition and the radio wave absorption sheet, and the results are shown in Table 1.
Dispersibility: Evaluated by visual inspection, and carbonyl iron is uniformly dispersed.
”Indicates that the carbonyl iron is slightly agglomerated and slightly inferior in dispersibility.
The state where ru iron aggregated and poor dispersibility was indicated as “x”. The poor dispersibility is
It means that workability is bad.
Hardness: The hardness of the radio wave absorbing sheet was measured according to JIS K6253 (type A).
Radio wave absorption performance: 5.8GHz radio wave absorption amount of radio wave absorption sheet is measured by reflected power method.
It was.
Ozone resistance: Measured according to JIS K6259 and evaluated for cracks after 100 hours.
I was worth it.

  From Table 1, the following is clear.

  That is, Comparative Example 1 is a natural rubber compounding system, which has good dispersibility of carbonyl iron, low hardness, flexibility and good radio wave absorption characteristics, but is inferior in ozone resistance. Moreover, since the comparative example 2 has few compounding quantities of carbonyl iron, it is inferior to electromagnetic wave absorption performance. On the other hand, in Examples 1 to 4 blended with natural rubber and ethylene propylene rubber, all of dispersibility, hardness, radio wave absorption characteristics, and ozone resistance can be satisfied. Further, with respect to the radio wave absorption characteristics, a better result was obtained for the natural rubber compounded one. In addition, although Example 5 is an ethylene propylene rubber compounding system, its dispersibility is poor and its flexibility is poor, but good results are obtained in radio wave absorption characteristics and ozone resistance.

Examples 6-11
A rubber composition for a radio wave absorber is prepared with the rubber composition shown in Tables 2 to 6, and the rubber composition for a radio wave absorber is vulcanized to obtain a radio wave absorber sheet having a thickness shown in Tables 2 to 6. It was. In Examples 6 to 11, all components were added and kneaded at one time (one-step kneading).

  Each radio wave absorption sheet was evaluated in the same manner as in Example 1 for hardness, radio wave absorption performance, and ozone resistance, and the results are shown in Tables 2-6.

  From Tables 2 to 6, it can be seen that according to the rubber composition for a radio wave absorber of the present invention using a weather resistant rubber as a rubber component, a radio wave absorbing sheet excellent in radio wave absorption characteristics and ozone resistance can be obtained.

Claims (13)

A rubber composition for a radio wave absorber obtained by blending 200 to 1500 parts by weight of carbonyl iron with 100 parts by weight of a rubber component,
A rubber composition for a radio wave absorber, comprising 5 parts by weight or more of weather resistant rubber in 100 parts by weight of the rubber component.   2. The group according to claim 1, wherein the weather resistant rubber comprises butyl rubber, chloroprene rubber, ethylene propylene rubber, ethylene propylene diene rubber, chlorinated polyethylene rubber, acrylonitrile butadiene rubber, chlorinated butyl rubber, brominated butyl rubber and chlorosulfonated polyethylene rubber. A rubber composition for a radio wave absorber, wherein the rubber composition is one or more selected from the group consisting of:   The rubber composition for a radio wave absorber according to claim 1 or 2, wherein 20 parts by weight or more of weather resistant rubber is contained in 100 parts by weight of the rubber component.   The rubber composition for a radio wave absorber according to any one of claims 1 to 3, wherein the rubber component is composed of a weather resistant rubber and a general-purpose rubber other than the weather resistant rubber.   5. The rubber composition for a radio wave absorber according to claim 4, wherein the general-purpose rubber is one or more selected from the group consisting of natural rubber, styrene butadiene rubber, isoprene rubber, and butadiene rubber.   6. The rubber for an electromagnetic wave absorber according to claim 5, wherein the rubber component includes natural rubber and ethylene propylene rubber in a ratio of natural rubber / ethylene propylene rubber = 95/5 to 5/95 (weight ratio). Composition.   7. A radio wave absorber rubber according to claim 6, wherein said rubber component contains natural rubber and ethylene propylene rubber in a ratio of natural rubber / ethylene propylene rubber = 80/20 to 55/45 (weight ratio). Composition.   The rubber composition for a radio wave absorber according to any one of claims 1 to 7, further comprising 1 to 10 parts by weight of carbon black with respect to 100 parts by weight of the rubber component.   The rubber composition for a radio wave absorber according to any one of claims 1 to 8, wherein 300 to 1000 parts by weight of carbonyl iron is blended with 100 parts by weight of a rubber component.   The rubber composition for a radio wave absorber according to any one of claims 1 to 9, wherein an average particle diameter of the carbonyl iron is 1 to 10 µm.   The rubber composition for a radio wave absorber according to any one of claims 4 to 10, obtained by adding and kneading a filler such as carbonyl iron to a weather resistant rubber and then adding and kneading a general-purpose rubber. .   The rubber composition for a radio wave absorber according to claim 11, obtained by adding and kneading a filler such as carbonyl iron to ethylene propylene rubber and then adding and kneading natural rubber.   13. A radio wave absorption sheet obtained by molding the rubber composition for a radio wave absorber according to claim 1 into a sheet shape.